A communication link may be described in the form ofy=H(x)  (1)where y is the received signal, x is the transmitted signal, and H represent the effects of the channel. In general y, H, and x are multi-dimensional with different dimensions representing time, frequency, receive antenna etc, and H(x) represents a general function of x. For simplicity it can be assumed that H can be represented by a M×N matrix, with y being a M×L vector, and x being a N×L vector, appreciating that this concept may be applied to more general environments. For illustrative purposes, we shall assume that the link represents a Multi-In-Multi-Out (MIMO) environment with N transmit antennas, M receive antennas, and L layers being transmitted.
In order for the receiver to coherently demodulate the transmitted data, knowledge of the channel H is needed. One method to achieve this knowledge is through the transmission of pilot symbols. These pilots can be either common, in which all/many mobile terminals can view them and determine the channel, or dedicated in which the channel knowledge is only available to a select group of mobile terminals. In many conventional wireless systems utilizing the common pilot scheme, the number of common pilots equals the number of transmit antennas, while in many conventional wireless systems utilizing the dedicated pilot scheme, the number of dedicated pilots equals the number of layers of data transmission.
In some cases, the transmitted signal x includes a pre-coded version of a data symbol s, pre-coded with a pre-coding matrix F chosen from a group of predefined matrices that is commonly called a codebook {F}. In some cases a receiver, for example a mobile terminal, tells a transmitter, for example a base station, which pre-coding matrix to use. For FDD (frequency division duplex) air interfaces the information identifying a pre-coding matrix may be fed back through either channel sounding approaches or codebook index approaches. TDD (time division systems) may also use the codebook based approach. A detailed example of an approach to pre-coding for MIMO transmission is described in it. J. Love, et al, “Limited Feedback Unitary Pre-coding for Spatial Multiplexing Systems”, IEEE Trans. Inf. Theory, vol. 51, no. 8, pp. 2967-2976, August 2005.
Codebook index feedback involves the receiver signalling to the transmitter an index of which pre-coding matrix to use (so-called codeword index). There are a plurality of indexes each corresponding to a respective pre-coding matrix. One problem, however, is that codebook index feedback approaches use a large amount of uplink radio resources.
An example of the common pilot approach is shown in FIG. 1 for a two transmit antenna case. In FIG. 1 (and FIG. 2 discussed below), the horizontal axis 210 is frequency (OFDM (Orthogonal Frequency Division Multiplexing) sub-carriers) and the vertical axis 212 is time (OFDM symbols). Each small circle represents a transmission on a particular sub-carrier over a particular OFDM symbol duration. In locations 214, pilots are transmitted by a first transmit antenna Tx-1, and in locations 216, pilots are transmitted by a second transmit antenna Tx-2. Remaining locations are available for data transmission by both antennas. In the illustrated example, data includes pre-coded data 218 for a first UE (UE-1), which may, for example, be a mobile terminal, and pre-coded data 220 for a second UE (UE-2), which may also be a mobile terminal. Typically, the pre-coding applied for pre-coded data 218 will be different from that applied for pre-coded data 220. With the common pilot approach, the same pilots are used for both UEs and are not pre-coded.
An example of the dedicated pilot approach in an OFDM signalling scheme is shown in FIG. 2 for a two transmit antenna case. In locations 222,224, dedicated pilots specific to a first UE are transmitted by a first transmit antenna Tx-1 and second antenna Tx-2 respectively. In locations 226,228, dedicated pilots specific to a second UE are transmitted by a first transmit antenna Tx-1 and second transmit antenna Tx-2 respectively. Remaining locations are available for data transmission by both antennas. In the illustrated example, data includes pre-coded data 230 for a first UE (UE-1), and pre-coded data 232 for a second UE (UE-2). Typically, the pre-coding applied for pre-ceded data 230 will be different from that applied for pre-coded data 230. With the dedicated pilot approach, different pilots are used for each UE in the sense that they are pre-coded using the same pre-coding matrix as used for the data for each user.
In general, since both pilot and data go through the same channel, a dedicated pilot scheme is more resilient to codeword index feedback error than common pilot based schemes.
Both pilot schemes have their own strengths and weaknesses. Common pilots overhead grows linearly with the number of transmit antennas, while the performance only increases proportionally to the logarithm of the number of transmit antennas. However, the overhead for each common pilot is much lower, as compared to the overhead associated with a dedicated pilot, as a common pilot can be shared between many users. Dedicated pilots have significantly higher overhead per layer; however the overhead increases linearly with the number of layers, which is the same rate as the increase in performance.
In addition, common pilots, due to their constant availability to most/all mobile terminals may be utilized for measurements/feedback, although the overhead for common pilots must be paid by all users even if they receive no benefit from them.
In a dedicated pilot based scheme, pilots can be pre-coded, and hence have the same channel matrices as data. One problem, however, is that since each UE (mobile terminal) trying to communicate with a base transceiver station (BTS) does not know what pre-coding matrix is being used by other UEs, the UE is typically unable to monitor the channel. More specifically, they do not know which pre-coding matrix is being used, do not know the rank of the current channel, cannot estimate per-layer based signal to interference noise ratio (SINR), and are unable to do channel dependent scheduling, to name a few examples.